Color television transmission systems used throughout the world are based on or derived from signal specifications originally defined in the United States by the National Television Systems Committee (NTSC). Such systems, which will be referred to herein as NTSC-type systems, include the NTSC format system used in the United States and the well-known PAL and SECAM systems used abroad. These systems utilize composite color television signals comprising a wide band monochrome signal and a plurality of chrominance signals.
The wideband monochrome signal, generally represented by the term Y', is typically a combination of three primary color signals, e.g., red, blue, and green, which have been precorrected for the power law gamma characteristic of typical display tubes. The presence of pre-correction in the constituents of a signal is conventionally indicated by designating the signal with a prime ('). The monochrome signal is typically of the form Y'=.SIGMA.A.sub.c C'=A.sub.r R'+A.sub.g G'+A.sub.b B', wherein C' represents any gamma-corrected primary color signal, A.sub.c, A.sub.r, A.sub.g and A.sub.b represent respective nominal relative luminance coefficients for primaries, and R', G', and B' represent the gamma-corrected color signals for primaries red, green, and blue, respectively. The monochrome signal Y', as defined herein, should not be confused with the colorimetric luminance Y which is a corresponding combination of the uncorrected primary signals, nor should it be considered equal to a gamma-corrected luminance signal because in the monochrome signal it is the individual primaries which have been corrected, not the entire combination, i.e., Y'=.SIGMA. A.sub.c C' is not uniquely related to Y=.SIGMA.A.sub.c C for typical gamma correction exponents.
The chrominance signals in NTSC-type systems typically comprise signals representing the difference between a gamma-corrected primary color signal and the monochrome signal or a linear combination of such color difference signals. Specifically, color difference signals can be generally represented by the term (C'-Y').sub.L wherein C' represents any gamma-corrected primary. The subscript L serves as a reminder that the chrominance signals are typically transmitted with a bandwidth which is relatively reduced as compared with the Y' signal and that it may be further bandwidth limited in the receiver.
Typical NTSC-type transmission systems are designed to transmit Y' in its full wide bandwidth and linear combinations of the chrominance signals in reduced bandwidth. In the United States, for example (R'-Y') and (G'-Y') are transmitted in linear combination signals designated the I' chrominance signal and the Q' chrominance signal. While the I' and Q' signals have somewhat different bandwidths, each substantially narrower than that of the Y' signals, the excess portion of the relatively wider bandwidth I' signal is often lost at the receivers, most of which are designed for equiband operation.
Conventional receivers use either equiband chrominance for all axes, or use in varying degrees the added intermediate bandwidth I' signal transmitted as a single-sideband component.
Some receivers use simplified approximations to the nominal I' passbands, while other receivers use wideband equiband systems. In order to shorten the chrominance transient epoch, these receivers accept erroneous chrominance components nominally from the single-sideband I' components, and they variously proportion these erroneous components between the I' and Q' channels.
This specification will present the equations and circuit means relative to the substantial chrominance improvements of this invention, first for processing of equal band signals and then also for processing of I' and Q' chrominance signals of unequal bandwidths.
Common NTSC-type receivers demodulate and matrix the received chrominance signals into a plurality of reduced bandwidth color difference signals (C'-Y').sub.L. The receiver then effectively adds the monochrome signal Y' to each chrominance signal in order to derive a plurality of signals which include, respectively, the low frequency components C.sub.L ' of the primary color signals generated at the color camera and a combined high frequency component. The low frequency primary color components are sometimes referred to as the large area color signals. The high frequency component, Y.sub.H ' is generally referred to as the mixed highs signal because it is transmitted and displayed only as a combination of the high frequency primary color components.
It has long been recognized that conventional NTSC-type receiving systems exhibit a number of visible color infidelities upon display, particularly in regions of high color saturation wherein one or more color primaries have low or zero values of local average color. When conventional NTSC-type receiver displays are compared against a reference display in which all of the primary color signals have a wide bandwidth comparable to that of Y', visible color infidelities, such as inadequate highs, overmodulation, rectification, and desaturation, can be observed in regions of high color saturation. These infidelities are clearly visible on modern displays as resolution and luminance errors, chromaticity smear, local desaturation, and spurious low frequency color components.
In regions of significant color saturation, a conventional color television display will typically exhibit: (1) a loss in detail due to inadequate high frequency components in the one or more strong primary colors and (2) over-modulation and rectification due to an excess of high frequency signal components in the one or more weak primary colors. The simplest example is that of a single saturated primary. In such case, the transmitted high frequency signal available for that primary solely from the high frequency signal Y' is too small, while the same Y' signal components are excessive in the other primary colors. More generally, similar infidelities generally occur and tend to be visible whenever the local color deviates significantly from white. Such infidelities are clearly visible on modern color television displays as resolution and luminance errors, chromaticity errors, and sometimes as spurious low frequency errors and local desaturation.
While there has been a widespread recognition that the conventional reception and display of NTSC-type signals produces the above-described color infidelities, none of the receiver correction circuits proposed in the prior art has provided satisfactory results. Typical prior art proposals for reducing saturation distortions have allocated the largest portion of the fault to the use of a Y' signal on transmission instead of a true luminance measure, such as a gamma-corrected Y signal. Accordingly, these proposals have included the proposal to change the transmitted signal from Y' to Y to the inverse-gamma power and various other proposals to otherwise precorrect the transmitted monochrome signal. All such proposals have gone unaccepted in the industry because (1) they typically failed to provide adequate color correction; (2) they typically degraded image quality in other respects; and (3) they were, in many cases, unduly complex.
The specific problem of inadequate highs has been treated, but the proposed solutions have deteriorated the image quality in other respects. For example, some prior art receivers utilize an enhanced gain in the common mixed-high region of the monochrome signal. This approach, however, cannot provide the differential relative amplitudes needed in the individual primary colors, and degrades the display image by producing increased rectification and desaturation. It has also been proposed to modulate the common mixed highs by the ratio of the square of an estimated gamma-corrected luminance to the square of Y'. This proposal, also, fails to provide differential relative amplitudes and it would introduce a major increase in rectification and desaturation as well as generate spurious high frequency signals.